The present invention relates to a novel and improved servo feed system, particularly adapted for use in a wire cutting type electrical discharge machining operation. When there occurs the condition known as "gap short circuit condition", such as when there is contact or pending contact between the wire electrode and the workpiece or bridging of particles between those elements, it is necessary to perform one or both of the following operations. Machining current should be interrupted or substantially reduced pending resumption of normal machining, or servo feed control should be maintained to stop the advance of the workpiece relative to the electrode until the condition is alleviated. With wire electrode cutting where the path is frequently curvilinear or around sharp corners, it is not possible to provide an appreciable backup as between the electrode and the workpiece without further complicating the problem. Other types of wire electrode servo feed arrangements are known which provide the retracing of wire paths for a predetermined distance and then resumption of forward servo feed when gap short circuit is alleviated.
The present invention, as it will be seen to operate, provides an improved servo feed system which operates in such manner that the retracing of the prior cutting path is rendered unnecessary through the appropriate control exercised over the frequency of stepping pulses provided to the stepping motor or motors used in the system.
The present invention is particularly adaptable for use with those embodiments of electrical discharge machining power supplies in which the pulse generator, drive stage and output switches are transistors or other electronic switches, and in which the pulse generator is of the independent operating type, such as a multivibrator, oscillator or the like. While transistors are employed as the electronic switches in the preferred embodiment, the present invention is not limited to use in conjunction with such switches. By "electronic switch," we mean any electronic control device having a plurality of electrodes comprising at least two principal or power conducting electrodes acting to control the current flow in the power circuit, the conductivity of the power circuit being controlled by a control electrode within the switch, whereby the conductivity of the power circuit is regulated statically or electrically without the movement of any mechanical elements within the switch. Included within this definition by way of illustration, but not limitation, are vacuum tubes, transistors, semi-conductor control rectifiers, and the like.
Various prior art gap short circuit protection systems have been developed which provide for reduction of the machining current responsive to abnormal gap condition. An example of such a short circuit protection system is disclosed in our above mentioned U.S. Pat. No. 3,855,443. The system which is described in that patent is one which, responsive to a gap short circuit condition, lengthens the off-time and allows a predetermined series of pulses to be applied to the gap. The pulses provided to the gap during the gap short circuit condition are of the normal machining pulse on-off time and frequency. A series of pulses are thus applied and are then followed by a lengthened off-time interval several times the duration of the off-time of the first series. The system includes means for providing and presetting the number of the series of pulses which will be passed during the gap short circuit condition.
In a wire cutting apparatus, it has been found to be advantageous during the period of gap short circuit not only to reduce the current being provided to the gap but further to immediately influence and control the servo feed system by circuitry and means which are included in connection with the teachings of the present invention.